The only caveat to this is that it assumes the document already some content in the body, a single letter will do. I’m not sure if this can be avoided, I tried opening an empty document in the Productivity Tool and it suffers from the same error – “Cannot open the file: Archive file cannot be size 0”. In my sample it will fail on WordprocessingDocument.Open. Maybe in the case of an empty document you have to create a body first – a problem for another time :D.